Gas turbine engines serve as principle sources of power in air, marine, and industrial environments. In a gas turbine engine, air is compressed and mixed with a fuel to form a combustible fuel/air mixture. The fuel/air mixture is then burned to produce hot exhaust gas that expands across a turbine to produce power. As with all heat engines, the efficiency of a gas turbine engine is related to the maximum and minimum temperatures in its operating cycle. To increase the efficiency and performance of such engines, therefore, it is desirable to increase the temperature of the exhaust gas at the turbine inlet. The turbine inlet temperature of the exhaust gas in a typical gas turbine engine has increased from about 700.degree. C. in the early 1950s to about 1350.degree. C. in present day engines. The increase in turbine inlet temperature was made possible by advances in metallurgy and component cooling techniques.
As a result of high turbine inlet temperatures, turbine components operate under complex and demanding combinations of stress and temperature in a high-velocity gas stream. To withstand such conditions, components in the turbine, particularly the turbine blades, are typically made from nickel-based superalloys. Extensive experience has shown that such alloys provide good resistance to creep, fatigue, and most types of corrosion, which are the principle degradation mechanisms in the hot sections (i.e., the combustion chamber and turbine) of gas turbine engines. The superalloys, however, are vulnerable to hot corrosion, which causes the breakdown of the protective oxide scale ordinarily present on these materials. The breakdown of the protective oxide scale accelerates the rate of consumption of the underlying substrate. Hot corrosion can be promoted by various contaminants present in the fuel and air, such as vanadium (V) and sodium (Na).
Vanadium is not typically found in distillate fuels, such as jet fuels. Therefore, vanadium induced hot corrosion is not a major concern for aircraft gas turbine engines. Vanadium, however, often is present in residual fuel oils, such as those used in marine and industrial gas turbines, and in some crude oils. The vanadium is usually present as a porphyrin or other organometallic complex but inorganic compounds of vanadium also have been reported. During combustion of the fuel, vanadium reacts with oxygen to form oxides. The vanadium-oxygen system comprises at least four oxides, VO, V.sub.2 O.sub.3, V.sub.2 O.sub.4 (VO.sub.2), and V.sub.2 O.sub.5. The first three oxides are refractory materials that have melting points in excess of 1500.degree. C. As a result, they pass harmlessly through the turbine. V.sub.2 O.sub.5, however, has a melting point of about 670.degree. C. Therefore, V.sub.2 O.sub.5 is a liquid at gas turbine operating temperatures and easily deposits on the surfaces of hot components to cause corrosion.
Sodium vanadate forms when sodium salts, which are present in either the fuel or air (particularly in marine environments), react with vanadium oxides. The sodium vanadate phases flux the normally protective oxide scales found on nickel-based superalloys.
Early studies of vanadium hot corrosion recognized that the accelerated oxidation associated with the presence of liquid V.sub.2 O.sub.5 could be attenuated if the melting point of the reaction products could be raised above the temperature inside a gas turbine engine. Researchers found that certain compounds, such as metal oxides, react with V.sub.2 O.sub.5 to form refractory vanadates. To date, numerous additives have been evaluated for their effectiveness in inhibiting vanadium hot corrosion. Currently, magnesium-containing compounds (e.g., MgSO.sub.4) are widely used in the industry because they can decompose to magnesium oxide (MgO), which in turn reacts with V.sub.2 O.sub.5 to form magnesium vanadate (Mg.sub.3 (VO.sub.4).sub.2). Magnesium vanadate has a melting point of 1150.degree. C. For reasons that are not well understood, however, MgSO.sub.4 is not particularly effective in inhibiting sodium vanadate corrosion. In addition, sulfur, such as from sodium sulfates in the compressor and SO.sub.2 from the fuel, greatly reduces the effectiveness of the MgO formed from MgSO.sub.4 because MgO reacts preferentially with the sulfur to form MgSO.sub.4 rather than with V.sub.2 O.sub.5 to form magnesium vanadate.
As a result, there is a need for a vanadium corrosion inhibitor that also is effective in the presence of sulfur and against sodium vanadate corrosion.